Sync separation and automatic gain control



Sept. 15, 1953 l. TOURsHoU 2,652,450

SYNC SEPARATION AND AUTOMATIC GUN CONTROL Filled May 29, 1948 W J ,fg/f

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Patented Sept. 15, 1953 SYNC SEPARATION AND AUTOMTC' GAIN CONTROL Simeon I. Toursh'ou; Philadelphia, Paf., assig-nor to Radio Corporation of America, acorporation of Delaware Application May 29, 1948, Serial -Noi S'OLO? (Cl. TTS- 7.3)

3 Claims. 1

The" present 'invention' relates-'itc television 1re;- ceiv'er's and more`v particularly,I but not necessarily exclusively, to novelapparatusin/'atelevision re'- cei'ver 'forobtainin'g' improved' separation of sync signals combined with improved 'means' for' controllingfthe'gainof onelo'r more receiver'ampliners.

The general `aim or object-ofthe present invention is "to provide `.ntrv'el means'for separating synci'pulse's from' a' composite video signal and also; if desired, for' developing a control voltage proportional-tothe signal amplitude 'for' automatic gain control (A. G C.) purposes. In' ac cordancewith one aspect 'of the invention; a single tube'may perform thiefc'omplete signal separating' function' as well'a's serving: to' develop a' control voltage; In" another embodiment of the invention to'be disclosed herein, one tube performs a sepa'rat'ing and' control signal developing function and' anothercooperating tube periorr'n's'an addi providefnovel'mea'ns vfor employing degeneration K in the cathode circuit of a tube to assist in obtaining a signa-l separating.; result.

Alfurther'object' is to develop a direct current controlsignal as la result 'of the operation of the separating featurevof this'invention which in'- v'olves the? cathode-circuit of a tube.

A'f'urthr'a'irnfor"object of the invention, ree'rlred"t`o'abovefin broader terms, resides in the prcv'ision of time constant circuit `combinations' inthecathode circuits of afpair of related vacu-Y um"tubes whereby the'tubes `perform a separating functionin la novel manner.

A'iurther object isto provide ina novel VVmanner 'for' obtaining anampliied A. G. C. signal having-the` desired polarityl and amplitude.

'Other objects andadvantages of the present-invention will, of course, become apparent and -immediately'fsuggestvthemselves to those skilled in theart to-Whichtlie invention' is directed from aVv reading of the following. specication in connection with the accompanying drawings in which.:

Fig. 1 shows diagrammatically one suitableA form of television-apparatus: embodying the invention* and employing tvv'ou tubes whichI cooperate' inlaccordan'ce with the invention; and? Fig.' 2 shows'televisi'on apparatus having an other embodiment of the invention. y Referrin'gnow tothe drawings,l and more par-V ticularly toIFig; V1 `thereof, vthere is illustrated a portion v-ofatelevision receiver of a lsuitable kind, for example, off the superheterodyne type,*co1n prisingthe usual-nrstdet'ector, atunable oscillator,l and Yan intermediate frequencyl (I. F.) amplier, none of which is shown as they are not arpa-rt yofthe present invention. The signal receiving equipment, just mentioned, or its lequivalent, provides'a 4signal which is fed to a video demodulator or second detector 4l S-shown, by v/ay of'example, as adiode. The output of the second detector I il is shown as beinguphysicallyconnected to-tgie first stage it orv a video Yamplifier (not snown in detail). The videofamplifier may comprise several 'directly` coupled stages oramplification andy its `output may be applied by Way of a physically connected coupling arrangement to the control electrode of an image Vproducing tube (not'siiown') Whichvmay bea fkinescope having appropriate image producing-characteristics.

In the illustrative arrangemenuthe output from the I.YF.-amplier (notshown) is delivered tothe second detector |16I through a blocking condenser 24 and a tunable impedance, for example; thefinductanceV 26 vvhich is tunable by a movable magnetic corejZ of powdered iron or the like;V A condenser 29 assists in tuning the inductanceV 25.V It will'Y be understood that a rescnant'circuit mayv exist vand serve as the coupling-means betweenV the I. F. amplifier and the second detector lli. The resonant circuit, if it is presenty inthe receiver which embodies this invention, includes the' tunable inductance 26, the inter-electrode capacity of the last stage tube (not shown) of the I. F. amplier, and other circuit-capacities including the second detector it. Seriesand shunt-.peaking coils 3l land 32 are electivelyarranged in the detector circuit and the demcdulatedA output is applied by Way of a connection `Ziltothegrid of the tube i8. In the illustrative arrangement the coil 3l is included inthe connection Silybut itmay be connected between the anode'otV the diodel and the connection-oi-thecoil 32. The output of the second detectorfbeing taken from the anode of the tube it? provides that sync polarity. is -in the Anegative direction asshownconventionally by the waveform-38 adjacent the'connection 34'. The output signal from the second detector It includes bianking signa-lsl and'horizontal or line sync pulses 43, as well as the image signal portion M oi a composite-signal applied to the television receiver. It Willwbevunderstoodthat at appropriate intervals vertical or eldsync pulses (not shown) Will occur-at the lower frame or leld frequency.

The output of the tube |8 is or may be directly coupled to the grid of the tube of the next amplifier stage (not shown) through a physical or conductive coupling 45. Series and shunt peaking coils 46 and 41, respectively, and a resistor 48 be included in a coupling network between the tube i8 and the grid of the tube of the next stage if desirable or necessary in order to avoid loss of high frequency components of the signal. The signal appearing in the connection 45 is substantially a replica of the plate-current wave of the tube I8 but is a voltage wave 38a inthe posi.- tive direction.

In accordance with the invention, the output of the tube I8 appearing in a connection 52 as well as the connection 45 is applied to the control electrodes or grids 56 and 51 of what is in eect a pair of tubes. In the illustrative arrangement, a duplex tube 58 is shown which comprises two independent triode sections. A suitable tube is the type 6SN'7-GI but it will be understood that this type is cited only by way of example. A pair of tubes having separate envelopes may be used and they need not be triodes.

The tube section which includes the grid 56 also comprises an anode 6| and cathode 62. The tube section having the grid 51 also includes an anode 66 and a cathode 68. Resistors 12 and 13 are included in the connections to the grids 56 and 51 respectively.

The anodes 6| and 66 are connected through a resistor 16 to a suitable voltage source (not shown) as indicated schematically on the drawing. Electrodes of the sections of the tube 58 and a tube 18, to be described, are connected to points on the voltage source (not shown) having different values. Certain of the tube circuits shown in the illustrative embodiment have physical or conductive coupling to other tube circuits and convenient voltage values have been indicated on the drawing. The voltage values are assumed solely for illustrative purposes and for convenience in describing the invention and its operation.

For example, cathodes of the detector I6 and amplier tube I8 are indicated as being connected to a negative potential point on the voltage supply source. Solely by way of example, this voltage is indicated as being 120 volts. The anode of the tube |8 is connected through a resistor 19 which is indicated as being connected to a point on the voltage supply source which is positive with respect to the connection of the cathodes of the tubes I6 and I8. Solely by way of example, this voltage is indicated as being +225 volts. However, a path to ground is provided from the anode of the tube |8 through the coils 48 and 41 and the resistor 48. One function of the resistor 19 is to apply a positive voltage on the grids 5S and 51 of the tube 58.

The inductor 41 is grounded and the anode of the tube I8 is positive with respect to its cathode since the latter is connected to a negative point on the power supply. The method of connecting voltage supply sources and the like, and tube electrodes is by now well known, and it is believed that the diagrammatic indication given in the drawing will be sufficient to indicate to those skilled in the art the nature of the tube circuits employed in the receiver which are illustrative only.

Both sections of the tube 58 have a combined function, the result of which is separation or partial separation of sync signals from the video signals. One section of this tube, in operation as being volts.

as a signal separator, provides a voltage which bears such a relation to the received signal that it may be utilized as an A. G. C. signal. The relationship of the parts to provide the combined function will be discussed rst.

The circuit for the cathode 62 includes a resistor 83 and a condenser 86. The time constant of this resistor-condenser combination 83 and 86 is sufliciently long to require anode or plate current flow for the full duration of each vertical sync signal. This section of the tube 58, therefore, acts as a partial sync separator for the vertical sync signals. The anode or plate current load for the vertical sync signals includes resistors 88, 89, 16 and a capacitor 92. The resistor 88 and the capacitor 92 act as a filter for removing the horizontal or higher frequency sync signals from the output of the vertical separator.

The section of the tube including the anode or plate 66 acts as a cathode follower to charge a capacitor |82 nearly equal to the positive peaks of the video signal. The resistor 16 acts as a plate load for the horizontal sync signals to provide partially separated horizontal sync. time constant of the cathode circuit is such that the lower frequency or vertical sync signal is del generated as far as the plate circuit is concerned. To accomplish this, the time constant of the resistor-condenser combination is short, being only several times the period of the horizontal line. The cathode circuit is completed through resistors |04, 08 and a variable resistor |08. The cathode is returned to a point of negative potential on the voltage supply source (not shown). Solely by Way of example, this point is indicated The resistor-condenser combination referred to above includes the condenser |02 and the resistors |04, |06 and the included portion of the resistor |08. The short time constant of the resistor-condenser combination in the circuit of the cathode 68 provides that noise or variation interference sets up ther cathode bias to a higher value than the signal for a very short time only and does no serious harm to either the `horizontal scanning or the A. Gr. C. to be described.

Since the cathode time constant is short, the circuit is highly degenerative to the vertical sync signals. That is to say, the cathode capacitor charges up to the peak of the Vertical sync inV a few microseconds and no further plate current flows during the remainder of the Vertical sync. The separate tube section comprising the cathode 62 serves to give the full vertical sync signal as explained above. The output of the two sections of the tube 58 is combined across the resistor 16 and is furnished by Way of a coupling condenser I |2 to apparatus to be controlled by the separated, or partially separated sync signal. This apparatus, just mentioned, may be a scanning oscillator or a deflection A. F. C. system.

The resistor |04 is the principal component of the R. C. circuit by reason of the presence of a capacitor ||4. The variable resistor |08 serves as a level setter for the A. G. C. signal developed by the section of the tube 58 acting as a cathode follower.

The previously mentioned tube 18 serves as a direct current amplier for the A. G. C. signal developed during operation of the section of the tube 58 which includes the cathode 68 as explained above. The grid ||8 of the tube 18 is connected at the junction of the resistor |04, the

resistor it@ and the capacitor H.. The cathode lil is returned to a point or negative voltage on the power supplysource (not shown). Solely by Way of example, the voltage at this point is indicated as being volts negative. The anode E22 is connected to a network comprising series resistors 520.1, E25, E26, and lil The point of connection o- F the ano-:le to this network is at the junction point of the resistors lili and mit. Resistor ill has a very high value and its free end is connected to the D. C. potential source (not shown) at a point of relatively high positive potential. Solely by way oi example, this'point is indicated as being +22@ volts. The resistor lill as stated previously is connected to ground. A condenser ltd is connected between the end of the resistor iZ'l and ground.

A connection 432 is taken from between the resistors lill and iZli which is, or may be, connected to the I. F. am'iliiier (not shown). A connection i3@ is available for connection to the R. F. stages (not shown) of the receiver ampliiier. An A. G. C. filter condenser itt is connected to the anode REZ oi the ampliiier tube lt.

In operation of the apparatus of Fig. l, the demodulator iS provide's'the demodulated video signal and the amplifier tube i8 arnpliiies and inverts this signal so that sync is in the positive direction. The circuit of the cathode t3 of the tube dii serves as a cathode follower charging the condenser lili' to the peaks or nearly to the peaks of sync. lBi-rect current voltage, proportional to the received signal amplitude can be taken .from the cathode. This direct current voltage, which is in the nature of a control voltage, is amplified by the tube 'it and inverted so that on strong received signals, the output of the tube it is more negative and is, therefore, in the proper direction to serve as an A. G. C. voltage.

The time constant for the circuit of the cathode [it is short and plate current flor-1s only during peaks of sync. Since the cathode time constant is short, the circuit is highly degenerative to vertical sync which is a signal of rela1 tively long duration.

The tube section including the cathode 62 has a cathode circuit time constant long enough to require plate current to iiow for the full duration of the vertical sync signal. 'l'.he separated vertical sync is nltered by condenser 92 and combined with the output of the other tube section Which develops the horizontal sync.

ci drawings discloses a further enibodiment oi the invention in which a single tube made to serve the urpose or obtaining the full vertical sync. Tube lilo performs the function of tube ifi i Fig. l and it will be understood that the demodulated composite signal is applied to the grid Mil of this tube. The output circuit for this tube is shown conventionally and includes a tap or branch Hifi which feeds other stages (not shown) of the video ampliner. The output signal, designated by the reference character since it is substantially the same signal as that appearing in l., is applied. to the grid ille of the tube ills. The cathode circuit for the cathode l5@ of this tube includes an R. C. combination comprising a resistor it@ and a condenser 52. The time constant of this R. C. combination is relatively short. In this'. and other respects it corresponds to the comm bination of the resistor lilil and the condenser |102 of Fig. l. Since this time constant is short,

PIL

the-'circuit is'highly degenerative to the vertical sync signals.- The capacitor lili therefore charges upto the peak offthe vertical sync signal in a few 'lnieroeseoonds and no further plate current ilows `during the remainder ofthe vertical'sync pulse. The tube lill? develops cathode biasl nearly equal to' the positive peaks of the video signal and plat-c current flows only during peaks of thereceive'd signal. These pe'alrs are the llorisontal sync vpulses and sync pulses of voltage appear on the plate.

ln the embodiment of 2, full vcrti'calsync is obtainedby vacoinoination` of lou7 frequency boost" in the plate'circuit oi the tube which increases the plate loadffor low frequency cornfponents vcontained inthe vertical sync. Alsoa resistance-inductance shunt is connected in the cathode circuit to reduce the cathod'cim'pedance for the lower frequency longer duration vertical sync pulses and which also reduces the degenere ative eiectof the cathode circuit 'on the vertical sync signals.

More in detail, and by Way of example, they low frequency boost feature is provided by a resistor 153 and a condenser its. These elcments are so proportioned With respect to each other and the frequency of the vertical sync signal that they increase the plate load for the vertical sync signal. The plate 55 of the tube Hi8 is connectedto a suitable voltage supply source (not shown) by Way of a load resistor l and the resistor 53. Separated sync sig nais are recovered in an output connection H59 to be available for use in any desired manner.

rThe shunt, mentioned above, around the R. C. combination is providedby a resistor It! and an inductance E62" connected in series. This series combination is shunted around the l. C. combination l t l and 52.

The A. G. C. signal, developed in operation oi the tube Hi8, isV taken from the cathode l5@ and iltered by a resistor its and a condenser ld and is applied to the grid itil of an arnpliiier tube 69 by way of a resistor lll. The outputl of this tube may be connected to the desired amplifier grids of the receiver ampliers (not shown) Havingnow describedthe invention, what is claimed and desired to be secured by the Letters Patent is the following:

l. In television apparatus having automatic gain control means and wherein high frequency and low frequency sync signals and image signals are received as a composite signal in which the sync signals are of greater amplitude than the image signals, means for detecting said composite signal, electric discharge tube means for separating said low frequency sync signals from said image signals, said electric discharge tube means comprising at least an anode, a cathode, and a control electrode, means to couple the composite output of said detecting means to said control electrode, a parallel resistor and condenser combination in circuit with said cathode, the time constant or said resistor and condenser combination being long so as to require plate current to flow for the full duration of each low frequency sync signal, a source of direct current Voltage for said anode, a load impedance included in a series circuit from said source to said anode, coupling means to recover said low frequency sync signals appearing across said load impedance, a second electric discharge tube means for separating said high frequency sync signals, said second electric discharge tube means comprising at least a cathode, an anode, and a control electrode, means to couple said output of said detecting means to said second control electrode, a second parallel resistor and condenser combination in circuit with said second cathode, the time constant of said second resistor and condenser combination being comparable to several times the period of the high frequency sync signals, a load impedance included in a series circuit from said source to said second anode, coupling means to recover said high frequency sync signals appearing across said last-named load impedance, and means including said second resistor-condenser combination for deriving from the cathode of said second tube a voltage proportional to the amplitude of said higher frequency sync signals, said Voltage being adapted to control said automatic gain control means.

2. In television apparatus wherein high frequency and low frequency sync signals and image signals are received as a composite signal in which the sync signals are of greater amplitude than the image signals, means for detecting said composite signal, electric discharge tube means for separating said 10W frequency sync signals from said image signals, said electric discharge tube means comprising at least an anode, a cathode, and a control electrode, means to couple the composite output of said detecting means to said control electrode, a parallel resistor and condenser combination in circuit with said cathode, the time constant of said resistor and condenser combination being long so as to require plate current to now for the full duration of each low frequency sync signal, a source of direct current voltage for said anode, a load impedance included in a series circuit from said source to said anode, coupling means to recover said low frequency sync signals appearing across said load impedance, a second electric discharge tube means for separating said high frequency sync signals, said second electric discharge tube means comprising at least a cathode, an anode, and a control electrode, means to couple said output of said detecting means to said second control electrode, a second parallel resistor and condenser combination in circuit with said second cathode, the time constant of said second resistor and condenser combination being comparable to several times the period of the high frequency sync signals, a load impedance included in a. series circuit from said source to said second anode, coupling means to recover said high frequency sync signals appearing across said last-named load impedance and wherein at least a portion of one of said load impedances comprises a portion of the other of said load impedances, whereby said separated sync signals are adapted to be combined in said common load impedance.

3. Television apparatus as set forth in claim 2 which comprises means including said second resistor-condenser combination for deriving from the cathode of said second tube a voltage proportional to the amplitude of said high frequency sync signals adapted to control said automatic gain control.

SIMEON I. TOURSHOU.

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